This invention relates to a terpolymer for inhibiting the formation of scale on reactor surfaces that are in contact with polymerizing vinyl monomers. In particular, it relates to a method of making the terpolymer without an acid catalyst and to a solution of the terpolymer, a salt, and a base.
When vinyl monomers are polymerized, scales of the polymer adhere to the reactor walls and to reactor components, such as stirrers. The deposition of this scale reduces the yield of the polymer, reduces heat transfer efficiency during heating and cooling of the reactor, and lowers monomer efficiency and general process control. Some of the scale flakes off during polymerization of the monomer and mixes with the rest of the polymer, which makes the polymer less processable and reduces its quality. For example, when vinyl chloride monomer (VCM) is polymerized, scales of polyvinyl chloride (PVC) can flake off and form xe2x80x9cfish eyesxe2x80x9d xcfx80in the PVC.
Cleaning the reactor to remove this scale is normally required after each batch, resulting in considerable reactor downtime as well as increased cost in producing the polymer. Because VCM can become entrapped in the scale, the physical removal of the scale by an operator could result in the exposure of the operator to the harmful monomer.
EP 343,706 teaches that polythioureas of the general formula [NHRNHC(S)]n (where R is an arylene group substituted by a carboxylic acid group) can be used to reduce scale formation in vinyl chloride polymerization reactors. JP 01,217,016 discloses a scale preventing compound comprising an alkali metal xanthogenate, polyethyleneimine, and a dialdehyde. In JP 59,32,482, dithiocarboxylic acids or salts containing OC(xe2x95x90S)S or NC(xe2x95x90S)S groups and/or (RO)2P(S)SR derivatives are disclosed as scale preventing agents. U.S. Pat. No. 4,431,783 discloses that naphthol/formaldehyde condensates formed by a base-catalyzed process are effective scale-preventing agents vinyl chloride polymerization reactors. While many of these scale prevention agents are effective in preventing scale, they can alter the color of the resulting PVC, giving it a yellowish tinge. Customers of high quality PVC prefer a pure white or colorless PVC and regard any color in the PVC as an indication of inferior quality.
We have discovered that a terpolymer of an aromatic compound, a carbonyl compound, and a thiourea can be made without using an acid catalyst. By omitting an acid catalyst, the terpolymer can be made with less cost as not only is the cost of the acid catalyst avoided, but subsequent neutralization and disposal of the acid catalyst is also avoided. We have further found that the terpolymer is more effective in reducing scale when a solution of it, a base, and a salt is applied to the reactor components. The scale prevention agent of this invention is very effective in preventing the formation of scale on reactor components that come in contact with polymerizing VCM. Because there is little or no scale, there are fewer fish eyes. It is easily prepared from commercially available, inexpensive, non-toxic materials and can be applied in an aqueous medium, thereby avoiding the use of potentially harmful and flammable organic solvents. Quite unexpectedly, the scale prevention agent of this invention imparts little or no color to the resulting PVC.
The terpolymer used in the process of this invention is the condensation reaction product of an aromatic compound, a carbonyl compound, and a thiourea. The aromatic compound contains the group: 
where Q is OR or SR and R is hydrogen, alkyl from C1 to C15, or aryl, alkaryl, or aralkyl from C6 to C15. The Q group is preferably OR and the R group is preferably hydrogen as phenols are more readily available and work well. Preferred aromatic compounds include: 
where Z is 0 or NOH, each R1 is independently selected from OH, R, SO3R, and CO2R, and each R2 is independently selected from R1, SH, SR, haloalkyl from C1 to CO15, and alkoxyaryl, naphthyl, and polycyclic aryl from C6 to C15. Examples of suitable aromatic compounds include salicylic acid, 1 -naphthol, thiosalicylic acid, hydroxybenzenesulfonic acid isomers, 4-hydroxybenzoic acid, 2xe2x80x2-hydroxypropiophenone oxime, and 2xe2x80x2-hydroxypropiophenone; 2xe2x80x2-hydroxypropiophenone and 1-naphthol are preferred due to their low cost, low toxicity, and good performance in the terpolymer condensate. Mixtures of the aromatic compounds can also be used.
The carbonyl compound can be an aliphatic mono or dialdehyde, an aromatic aldehyde, or an aliphatic mono or diketone. Compounds, such as trioxane, that form the carbonyl compound under the reaction conditions, are also contemplated. Preferred carbonyl compounds have the general formula: 
where each R2 is independently selected as hereinabove defined and n is an integer from 1 to 10. Examples of suitable carbonyl compounds include acetaldehyde, glyoxal, benzaldehyde, dialkyl ketones such as acetone and methyl ethyl ketone, diaryl ketones such as benzophenone and ring-hydroxylated and sulfonated benzophenones, alkyl aryl ketones such as acetophenone, ring-hydroxylated acetophenones, and ring-sulfonated acetophenones, and formaldehyde and oligomers thereof, such as trioxane. Preferably, the carbonyl compound is formaldehyde or an oligomer thereof as those compounds have low cost, high reactivity, and are readily available. Mixtures of carbonyl compounds are also contemplated.
The thiourea has the general formula 
where each R3 is independently selected from NH2 and R2. Examples of suitable thioureas include thiourea, thiocarbamide, monomethyl thiourea, 1,3-dimethyl-2-thiourea, 1-phenyl-2-thiourea, semicarbazide, thiosemicarbazide, and thiocarbohydrazide. The preferred thioureas are thiourea and thiosemicarbazide because they work the best, are inexpensive, and are readily available. Mixtures of thioureas can also be used.
The terpolymer is made by preparing a mixture of its three components. The most preferred combination of the three components is a 1 -naphthol-formaldehyde-thiosemicarbazide (NFTS) terpolymer as it has the best combination of properties. For every mole of the aromatic compound, about 0.1 to about 10 moles of the carbonyl compound and about 0.1 to about 10 moles of the thiourea can be used. If less than 0.1 moles of the carbonyl compound is used, the terpolymers produced will have a lower molecular weight, and if more than 10 moles of the carbonyl compound is used, the terpolymer will have lower solubility. If less than 0.1 moles of the thiourea is used, the terpolymer will be less effective in preventing scale formation and, if more than 10 moles of the thiourea is used, the terpolymer will be less soluble. Preferably, for each mole of the aromatic compound, about 0.1 to about 5 moles of the carbonyl compound and about 0.1 to about 5 moles of the thiourea are used.
While the condensation reaction of the terpolymer components can be carried out in an acidic or basic medium, which also acts as the catalyst, preferably no acidic or basic catalyst is used as it has been found that none is needed. Mineral acids, such as hydrochloric acid and sulfuric acid, and organic acids, such as acetic acid, can be used as an acid catalyst and bases such as sodium hydroxide, potassium hydroxide, calcium hydroxide, magnesium hydroxide, barium hydroxide, sodium carbonate, potassium carbonate, and organic tertiary amines can be used as a basic catalyst. The strength of the acidic or basic catalyst can vary from about 1M to about 6M. If a catalyst is used, typically about 50 to about 500 wt % of it is used, based on the total weight of the terpolymer components.
The condensation reaction of the terpolymer components can be performed in water or in a polar organic solvent such as an alcohol (e.g., methanol, ethanol, propanol), a ketone (e.g., acetone, methyl ethyl ketone), an ester (e.g., ethyl acetate), or a dipolar aprotic solvent, such as sulfolane, tetrahydrofuran, N,N-dimethylformamide (DMF), N-methylpyrrolidone, cyclohexanone, dimethoxyethane, or diethyleneglycol dimethyl ether. Water is the preferred solvent because it is nonflammable and is not hazardous to the operator. Mixed solvent systems comprising combinations of solvents, including combinations of organic solvents with water, can also be used. It is preferable to choose a solvent system in which the terpolymer product is insoluble and precipitates.
The terpolymer can be prepared according to the method described in the Indian Journal of Chemistry, Vol 22A, pages 117 to 119, (1983), herein incorporated by reference. Typically, the aromatic compound, the carbonyl compound, and the thiourea taken together constitute from about 5 to about 50 wt % of the reaction system weight. The reaction can be effected by heating to temperatures of up to 150xc2x0 C. for up to 8 hours. Once the terpolymer has been prepared, it can be collected and purified if desired. While we do not wish to be bound by any theories, we believe that the NFTS terpolymer has the formula: 
where n is 1 to 15.
The terpolymer is preferably dissolved in a solvent to form a solution containing about 0.001 to about 20 wt % terpolymer. Solutions containing less terpolymer are less effective and solutions containing more terpolymer inhibit the polymerization of the VCM. Preferably, the solution contains about 0.01 to about 6 wt % terpolymer. Solvents that can be used to form the solution include, for example, water, alkaline water, and various organic solvents such as methylene chloride, tetrahydrofuran, dimethyl formamide, and dimethylsulfoxide. Up to about 20 wt % of the solvent can be an alcohol to help solubilize the terpolymer; water-miscible alkanols such as methanol and ethanol are preferred.
If a basic solution is used as the solvent, about 1 to about 5 wt % of a base (based on solution weight) is included in the solution. Less base is less effective in dissolving the terpolymer and more base is unnecessary; the preferred amount of base is about 0.5 to about 2 wt %. Suitable bases include NaOH, KOH, LiOH, and Ca(OH)2; the preferred base is NaOH because it is inexpensive.
The solution also includes a salt, which enhances the effectiveness of the solution in inhibiting scale formation. While any salt, including organic salts, can be used, the preferred salts are inorganic salts of a strong acid (e.g., HCl, H2SO4, HNO3, H3PO4) and a strong base (e.g., NaOH, KOH) as they are more effective; sodium chloride is especially preferred as it is inexpensive. The maximum amount of salt is the amount that can be dissolved in the solvent, typically about 15% for most salts in water. The amount of salt used in the solution should be at least about 0.1 wt %, as less is not very effective. Preferably, the amount of salt is about 8 to about 10 wt % of the solution.
Various other components can be added to the solution to enhance the performance of the terpolymer as an anti-fouling agent. For example, about 10 to about 500 wt % (based on solution weight) of an inorganic binder can be included in the solution to increase the binding of the terpolymer to the reactor surfaces. Examples of suitable inorganic binders include colloidal silica and alumina, titanium (IV) oxide, zirconium (IV) oxide, aluminum hydroxide, vanadium (V) oxide, and sodium silicate. The preferred inorganic binder is colloidal silica because of its low cost and good performance.
About 1 to about 100 wt % (based on terpolymer weight) of a surface active agent can also be included in the solution to increase the adhesion of the terpolymer to the reactor surfaces. Examples of suitable surface active agents include poly(vinyl acetate) (PVA) of varying degrees of hydrolysis, hydroxyethyl cellulose, hydroxypropyl cellulose, phytic acid, poly(vinyl formal), and gelatin. The preferred surface active agent is PVA of varying degrees of hydrolysis because of its low cost and good performance.
A preferred solution is about 0.01 to about 6 wt % terpolymer, about 0.1 to about 5 wt % base, about 0.1 to about 15 wt % salt, possibly an alcohol, and the rest water. The most preferred solution has a pH of about 12 to about 13 and comprises NFTS, aqueous NaOH, NaCl, organic solvent, inorganic binder, and a surface active agent.
The solution of the terpolymer can be applied to the reactor walls and to reactor components that are in contact with the VCM by any suitable means including, for example, spraying, flooding, painting, and brushing. After the application, it is not necessary to evaporate the solvent from the coating composition. Generally, the solids content of the coating on the reactor walls and components should be about 1xc3x9710xe2x88x925 to about 1xc3x9710xe2x88x923 g/square inch (about 1.6xc3x9710xe2x88x926 to about 1.6xc3x9710xe2x88x924 g/cm2) as less may not be effective and more is unnecessary.
The solid terpolymer can also be added directly to the polymerization composition. For emulsion polymerizations, the polymerization composition typically consists of water, vinyl monomer, a free radical initiator, and an emulsifier. For suspension polymerizations, the polymerization composition typically consists of water, vinyl monomer, a free radical initiator, and a suspending agent. Generally, about 0.0001 to about 0.01 wt % of the solid terpolymer is added to the polymerization composition, based on the weight of the monomer. Less terpolymer is ineffective and more terpolymer may inhibit polymerization of the monomer.
While the present invention is particularly suitable for polymerizations in an aqueous medium, such as suspension or emulsion polymerization, the terpolymer of this invention can also be used in gas phase or bulk polymerizations of vinyl monomers. The terpolymer can be used in both continuous and batch polymerizations.